This project is part of a general effort to understand the developmental mechanisms that construct the axon tracts of the central nervous systems of vertebrates. The present series of experiments will test the role of fasciculation in the initial formation of axon tracts. This research begins from the observations that transplanted optic and Mauthner axons will follow strikingly stereotyped growth routes when they are induced to form aberrant axon tracts in the CNS of amphibians. The proposed experiments will use HRP staining and light and electron microscopy to analyze the fasciculation of transplanted axons with other transplanted axons and with indigenous axons. In addition, transplants to late neural plate stage embryos will be used to produce aberrant innervation of neural tubes still devoid of indigenous axons. These experiments will test the ability of transplanted axons to form orderly patterns without local axons for fasciculation. Finally, Mauthner axons transplanted to the optic system will test the ability of axons (the Mauthner axons) to "overcome" local guidance forces (such as fasciculation) and to gain access to more appropriate pathways.